There are several mat cutters on the market one of which is manufactured according to U.S. Pat. No. 2,924,010, but these cutters generally do not have means to guide the tool for cutting circles in mats; and it is desirable that some kind of accurate guide should be provided for such mat cutters to circumscribe circles and cut them with as much exactness as is possible. It is the purpose of the present invention to provide an attachment for an already existing mat cutter which will quickly and easily cut circles.